1. Field of the Invention
This invention relates to industrial process instruments for displaying the values of measured process variables such as temperature, pressure, and flow rate. More particularly, this invention relates to indicators and/or chart recorders especially adapted for installation as part of the control panel of a process instrumentation system. In a preferred embodiment of the invention to be described, a combination indicator-and-recorder presents the values of up to three independent variables by means of so-called ribbon indicators, and simultaneously produces a permanent strip chart record of the values of those variables.
2. Description of the Prior Art
A wide variety of instruments have been used over the years for displaying and recording the measured values of industrial process conditions such as temperature, pressure, flow rate, etc. With the ever-increasing complexity of modern industrial processes, and the need to control the processes from a central station having an operator's panel display, there has been increasing need for reduction in size of the panel area occupied by each instrument, and in recent years the size of indicating and/or recording instruments has been reduced correspondingly. Also, the number of such instruments required for any given process has been reduced by arranging the instruments to indicate or record more than one variable per instrument.
So-called rectilinear-chart recorders are generally considered to be superior, and because of the relatively large pen-drive power requirements, rectilinear chart recorders commonly have employed servo motors to move the pen. In such equipment, the motor is controlled by an error signal representing the difference between the measurement signal and a pen-position feedback signal. The error signal is directed to an amplifier which in turns drives the servo motor, and thus the pen-drive power is not limited to that available from the measurement signal. Certain indicator/recorders presently offered use a pen drive of the stepping motor type, which has some inherent characteristics which are well suited to the pen drive function.
Experience with such indicator/recorders currently available indicates that improvements are needed to provide a fully satisfactory instrument. For example, achieving proper dynamic response of the pen motion for a given error condition is difficult, due in part to the incremental nature of the pen drive motion. In addition, operational problems are encountered when the pen is driven close to or into the limit positions at the ends of the full-scale pen stroke. Also, there is a need to simplify the pen drive apparatus to assure that it is adequately compact, light in weight, and reasonably inexpensive to manufacture. The present invention is aimed at solving or reducing such problems.